It is common to see portable digital devices becoming increasingly compact. Demand for these compact devices is very high as is the demand for accessories for such devices. A popular accessory used with such compact devices is a set of speakers. However, the size of the set of speakers is usually made to be compact in order to match the appearance of the portable digital devices. Unfortunately, the compactness of such speakers limits box volume and thus, prevents the use of a vented box system design. As such, closed box-small volume designs are often used for such speakers.
In the field of acoustics engineering, it is widely acknowledged that a low cutoff frequency of a loudspeaker system is affected by enclosure volume of the loudspeaker system. This is clearly demonstrated by the following equation:
      f    c    =            f      s        ⁢                  1        +                              V            AS                                V            B                              where ƒc=resonance frequency of enclosure, ƒs=driver resonance frequency, VAS=equivalent volume and VB=enclosure volume. Thus, it can be seen that the larger the enclosure volume, the lower the resonance frequency of enclosure (cutoff frequency).
Based on the aforementioned equation, it can be seen that the closed box-small volume designs would have inadequate low frequency reproduction.
Some of the aforementioned speaker systems do not have their own power supply. They draw power from the connected portable digital device to power speaker drivers, and correspondingly drains power from the portable digital device, significantly reducing a duration of usage for the portable digital device.